If most people think of anything when they hear the words “motor neuron disease,” they think of amyotrophic lateral sclerosis (ALS). However, there are other types of motor neuron disease as well. Fortunately, all motor neuron diseases are uncommon.
When we move, electrical signals are sent from the brain to the spinal cord along upper motor neurons. The nerve cells synapse in the anterior horn of the spinal cord, and then are sent out along lower motor neurons in peripheral nerves. Electrical signals traveling along these neurons signal for a muscle to contract, resulting in movement. Depending on certain physical exam findings, neurologists can determine where the problem is in the nervous system, and based off that, what type of problem is most likely.
Primary Lateral Sclerosis
Primary lateral sclerosis (PLS) is a disease of upper motor neurons, disrupting signals from the brain to the spinal cord. The cells in the cerebral cortex responsible for movement slowly die away. The result is a slowly progressive weakness associated with upper motor neuron signs, such as spasticity, rigidity, and increased deep tendon reflexes. Unlike amyotrophic lateral sclerosis, lower motor neuron findings, such as atrophy and fasiculations, are not as prominent. It is not certain just how common PLS is, but we believe it is less common than ALS.
Initially, PLS may be confused with the better-known ALS. Also, because ALS can begin with just upper motor neuron signs, neurologists usually wait for years before making a diagnosis of PLS. Even then, some cases of PLS will develop lower motor neuron findings, a sign that the disease is actually ALS. Other conditions, such as hereditary spastic paraparesis, will also need to be ruled out. Unlike ALS, PLS tends to progress more slowly, with patients commonly living up to 20 years with their symptoms.
Progressive Muscular Atrophy
In some ways, progressive muscular atrophy (PMA) is the opposite of primary lateral sclerosis. In PMA, only the lower motor neurons are affected, whereas in PLS, only the upper motor neurons are injured. While it is characterized by progressive weakness, no upper motor neurons signs, such as rigidity, are found in PMA. Like PLS, PMA is more rare than ALS, and has a better prognosis. It may take a long time to come to a diagnosis of PMA in order to rule out ALS as well as diseases like multifocal motor neuropathy (a form of peripheral neuropathy) and spinal muscular atrophy.
Progressive Bulbar Palsy
Progressive bulbar palsy involves a slow degeneration of the brainstem, which contains the nerves that control the face, tongue and throat. As a result, someone with progressive bulbar palsy will start having difficulty speaking, swallowing and chewing. Limb weakness may also become more evident as the disease progresses, with both upper and lower motor neuron signs. People with progressive bulbar palsy may also have uncontrollable and sometimes inappropriate outbursts of laughing or crying. It is not uncommon for people with progressive bulbar palsy to go on to develop ALS. Myasthenia gravis may also present in a similar fashion.
Polio is a virus that attacks motor neurons in the anterior horn of the spinal cord, resulting in paralysis. Thankfully, due to aggressive vaccinations, this virus has largely been eradicated. However, those who have had the disease sometimes complain of a weakness known as post-polio syndrome. This may be due to aging or injury causing the relatively few surviving motor neurons controlling the movement of a previously affected limb to die. The disorder only affects older people who have had polio in the past. It is usually not life-threatening.
Kennedy's disease is due to an X-linked genetic mutation that affects the androgen receptor. The disorder causes slowly progressive weakness and pain of the muscles closest to the torso. The face, jaw and tongue are also involved. Because it is X-linked, Kennedy’s disease generally affects men. Women with the genetic mutation are carriers, with a 50% chance of passing the gene on to their children. Women with the mutation may also suffer from minor symptoms, such as finger cramps, instead of more profound weakness.
Because the disease affects the androgen receptor, men with the disorder may also suffer from symptoms such as gynecomastia, testicular atrophy, and erectile dysfunction. The lifespan of people with Kennedy’s disease is usually normal, though as their weakness progresses they may require a wheelchair.
Spinal Muscular Atrophy
Spinal muscular atrophy is an inherited disease that predominantly affects children. It is caused by defects in the SMN1 gene, and is inherited in an autosomal recessive pattern. Due to this defective gene, not enough SMN protein is made, and this leads to degeneration of lower motor neurons. This leads to weakness and muscle wasting.
There are three main types of SMA, each involving children at a different age. SMA type 1, also called Werdnig-Hoffman disease, becomes obvious by the time a child is six months old. The child will have hypotonia (floppy muscles) and will not often move spontaneously. They will not be able to sit up on their own at the expected time. Due to difficulty with the airway and maintaining strength enough to breathe, most of these children die by the age of two.
SMA type II starts a little later, becoming apparent between the ages of 6 to 18 months. These children will not be able to stand or walk without assistance, and will also have difficulties with respiration. However, children with SMA type II typically live longer than those with Werdnig-Hoffman, sometimes living into young adulthood.
Type III SMA (Kugelberg-Welander disease) becomes apparent between the ages of 2 and 17 years. Children with this disorder may have some difficulty running or climbing steps. They may also have back problems, such as scoliosis. However, children with this disorder may have a normal lifespan.
To learn about spinal muscular atrophy, read here: SMA overview
There is no very effective treatment for any of the motor neuron diseases. Medical therapy focuses on controlling symptoms of the disease as best as possible. However, in order to know what symptoms to anticipate, as well as rule out other more treatable diseases, it is important to get the right diagnosis. Using their physical exam and other techniques such as electromyography, nerve conduction studies, and genetic testing when appropriate, neurologists can do a lot to help people with motor neuron disease.
AH Ropper, MA Samuels. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.
L.P. Rowland, Progressive muscular atrophy and other lower motor neuron syndromes of adults. Muscle Nerve, 41: 161–165. 2010